The process by which a photoresist mask is formed on a semiconductor substrate involves first coating the substrate with a thin layer of photoresist, exposing the resist with the desired pattern, and then developing the photoresist layer.
In performing this process, silicon substrates are placed on hot plates where they are baked at precise temperatures for precise periods of time. In a typical process, a clean substrate is initially subjected to a dehydration bake at 100.degree.-150.degree. C. to remove moisture. Then, after a thin layer of photoresist is applied to the substrate, the substrate is subjected to a "soft" bake at 90.degree.-120.degree. C. Again, after the photoresist has been exposed, the substrate is subjected to a post-exposure bake at 60.degree.-120.degree. C. Also, following a developing step, the substrate is subjected to a "hard" bake at 130.degree.-160.degree. C. to dry the substrate. After each of the foregoing heating steps, the substrate must be cooled to room temperature in order to assure a uniform process. Precise baking and cooling of silicon substrates is one of the most important processes used in the fabrication of very large scale integrated (VLSI) circuits.
All of these baking and cooling steps must be performed in a clean room which is temperature and humidity controlled and substantially free of dust and other particulate matter. In the prior art, the most common system for performing this process consists of a track arrangement in which the substrate is transported to successive stages in a sequence. This type of arrangement has a limited flexibility, since the substrates are locked in a fixed order.
In a less common type of arrangement, various hot plates and cool plates are positioned on two sides of a long central track and the substrates are transported between plates by a single robot which moves back and forth along the central track. Such prior art arrangements are extremely wasteful of floor area since no processing occurs in the track area.
In addition, in these prior art systems, if a single robot arm handles all the substrates, only one substrate may be serviced at any one time. Thus if two substrates are finished baking at the same time, only one of the substrates can be removed from the hot plate. The other substrate may be overbaked.
Also, in prior art systems, since a robot arm's end effector removes substrates from a hot plate, the end effector usually heats up over a period of time. When the hot end effector picks up a room temperature substrate having a recently coated photoresist layer, the photoresist layer may be heated non-uniformly in areas touched by the end effector. This may result in the photoresist coating being thicker in areas where the substrate was heated by the end effector. A 1.degree. C. variation in temperature can result in 20 .ANG. variation in thickness. Such variations are unacceptable since modern substrate processing must yield coatings of uniform thickness of 0.5 micron with variations no more than approximately 10 .ANG..
Furthermore, in the prior art, coiled vacuum lines are often used to supply vacuum for holding substrates in position. These vacuum lines, which extend through the equipment, may get in the way of robots, belts and other moving equipment and get chewed up and broken.
These problems are overcome in a thermal process module in accordance with this invention.